1. Field of the Invention
This invention relates to motor protection circuits of the type which respond to an overheating condition of a motor as sensed by a temperature sensor and effect the deenergization of the motor, and more particularly to a motor protection circuit which detects both open and short circuit conditions for the temperature sensor and prevents energization of the motor.
2. Description of the Prior Art
Known motor overheating protection circuits typically employ a variable resistance device, such as a thermistor embedded in the windings of the motor, to sense overheating of the motor windings and effect deenergization of the motor to prevent the windings from burning out. The thermistor is connected in a resistive bias network to generate a control signal which increases in correspondence with an increase in the motor temperature. A detecting circuit monitors the control signal and effects deenergization of the motor whenever the control signal reaches a set point value indicative of overheating of the motor. Since the detecting circuit is designed to respond only to an increase in the control signal, then any fault of the temperature sensor resulting in a decrease in the control signal would go undetected. For example, when a PTC thermistor is used, the detecting circuit would not respond to a short circuit condition for the thermistor or the wires which connect it to the detecting circuit. Conversely, when an NTC thermistor is used, the detecting circuit would not respond to an open circuit condition for the thermistor or its input wires.